1. Field of the Invention
The invention relates to dopamine receptors from mammalian species and the genes corresponding to such receptors. In particular, it relates to the human dopamine receptor D4. Specifically, the invention relates to the isolation, cloning and sequencing of the human D4 receptor gene. The invention also relates to the construction of eukaryotic expression vectors capable of expression of the human D4 dopamine receptor in cultures of transformed eukaryotic cells and the synthesis of the human D4 dopamine receptor in such cultures. The invention relates to the use of such cultures of transformed eukaryotic cells producing the human D4 dopamine receptor for the characterization of antipsychotic drugs.
2. Information Disclosure Statement
Dopamine is a neurotransmitter that participates in a variety of different functions mediated by the nervous system, including vision, movement, and behavior (see generally Cooper et al., 1978, The Biochemical Basis of Neuropharmacology, 3d ed., Oxford University Press, New York, pp 161-195). The diverse physiological actions of dopamine are in turn mediated by its interaction with two of the basic types of G protein-coupled receptors, D1 and D2, which respectively stimulate and inhibit the enzyme adenylyl cyclase (Kebabian & Calne, 1979, Nature 277: 93-96). Alterations in the number or activity of these receptors may be a contributory factor in disease states such as Parkinson's disease (a movement disorder) and schizophrenia (a behavioral disorder).
A great deal of information has accumulated on the biochemistry of the D1 and D2 dopamine receptors, and methods have been developed to solubilize and purify these receptor proteins (see Senogles et al., 1986, Biochemistry 25: 749-753; Sengoles et al., 1988, J. Biol. Chem. 263: 18996-19002; Gingrich et al., 1988, Biochemistry 27: 3907-3912). The D1 dopamine receptor in several tissues appears to be a glycosylated membrane protein of about 72 kD (Amlaiky et al., 1987, Mol. Pharmacol. 31: 129-134; Ninik et al., 1988, Biochemistry 27: 7594-7599). The D2 receptor has been suggested to have a higher molecular weight of about 90-150 kD (Amlaiky & Caron, 1985, J. Biol. Chem. 260: 1983-1986; Amlaiky & Caron, 1986, J. Neurochem. 47: 196-204; Jarvie et al., 1988, Mol. Pharmacol. 34: 91-97). Much less is known about a recently discovered additional dopamine receptor, termed D3 (Sokoloff et al., 1990, Nature 347: 146-151).
Dopamine receptors are primary targets in the clinical treatment of psychomotor disorders such as Parkinson's disease and affective disorders such as schizophrenia (Seeman et al., 1987, Neuropsychopharm. 1: 5-15; Seeman, 1987, Synapse 1: 152-333). The three different dopamine receptors (D1, D2, D3) have been cloned as a result of nucleotide sequence homology which exists between these receptor genes (Bunzow et al., 1988, Nature 336: 783-787; Grandy et al., 1989, Proc. Natl. Acad. Sci. USA 86: 9762-9766; Dal Toso et al., 1989, EMBO J. 8: 4025-4034; Zhou et al., 1990, Nature 346: 76-80; Sunahara et al., 1990, Nature 346: 80-83; Sokoloff et al., 1990, Nature 347: 146-151).
The antipsychotic clozapine is useful for socially withdrawn and treatment-resistant schizophrenics (see Kane et al., 1990, Nature 347: 146-151), but unlike other antipsychotic drugs, clozapine does not cause tardive dyskinesia (see Casey, 1989, Psychopharmacology 99: 547-553). Clozapine, however, has dissociation constants for D2 and D3 which are 3 to 30-fold higher than the therapeutic free concentration of clozapine in plasma water (Ackenheil et al., 1976, Arzneim-Forsch 26: 1156-1158; Sandoz Canada, Inc., 1990, Clozaril: Summary of preclinical and clinical data). This suggests the existence of dopamine receptors more sensitive to the antipsychotic clozapine than those known in the prior art heretofore.
We have cloned and sequenced such a human dopamine receptor which we term D4. The dopamine D4 receptor gene has high homology to the human dopamine D2 and D3 receptor genes. The pharmacological profile of this receptor resembles that of the D2 and D3 receptors but it has an affinity for clozapine which is tenfold higher. The present inventors envision that the D4 dopamine receptor disclosed as this invention may prove useful in discovering new types of drugs for schizophrenia that like clozapine do not induce tardive dyskinesia and other motor side effects.
We have also discovered that the D4 gene is polymorphic in the human population, having at least 7 different alleles that can be detected by restriction fragment length polymorphism analysis (see, Botstein et al., 1980, Am. J. Hum. Genet. 32: 314-331). This is the first receptor in the catecholamine receptor family which displays polymorphic variations in the human population. The observed polymorphism in dopamine D4 receptor genes may underlie individual differences in susceptibility to neuropsychiatric disorders such as schizophrenia and manic depression, as well as responsiveness to antipsychotic medication.